Mechanical and physical properties of protein-starch based plastics produced by extrusion and injection molding

Processing conditions, ingredient ratio, and moisture content were optimized for making soy protein/starch based plastics using a twin-screw extruder and an injection molding machine. A metering pump and a high speed mixer were used for ingredient mixing. The optimal processing temperature for injection molding was 130 degrees C, and the moisture content of extruded pellets was 10-14%. Processing effects were investigated by measuring the tensile properties and water resistance of specimens. Reduction of water and glycerol in mixtures increased the barrel pressure of the extruder. Mold release was improved by incorporating 0.25 parts tallow per 100 parts of solid material (soy protein and starch). The water absorption of the specimens was reduced by adding acids to adjust the pH to the isoelectric point of soy proteins (pH 4.5). Effects of storage at different relative humidities were studied. The processibility of pellets was stable after a 4-wk-storage period, despite some moisture loss. Injection-molded specimens, after being stored for up to 6 mon at dry conditions [50 and 11% relative humidity (RH)] at room temperature and for 4 wk in a 50 degrees C oven, showed no surface crack. However, humid (93% RH) storage at room temperature promoted fungal growth after storage for 3 mon, indicating that preservatives such as potassium sorbate and propionic acid were needed.